[3.13.y.z extended stable] Patch "word-at-a-time: avoid undefined behaviour in zero_bytemask macro" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    word-at-a-time: avoid undefined behaviour in zero_bytemask macro

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.3.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 137662128b238eeaa18fcfad1bb97acacfb90b2b Mon Sep 17 00:00:00 2001
From: Will Deacon <will.deacon at arm.com>
Date: Wed, 23 Apr 2014 17:52:52 +0100
Subject: word-at-a-time: avoid undefined behaviour in zero_bytemask macro

commit ec6931b281797b69e6cf109f9cc94d5a2bf994e0 upstream.

The asm-generic, big-endian version of zero_bytemask creates a mask of
bytes preceding the first zero-byte by left shifting ~0ul based on the
position of the first zero byte.

Unfortunately, if the first (top) byte is zero, the output of
prep_zero_mask has only the top bit set, resulting in undefined C
behaviour as we shift left by an amount equal to the width of the type.
As it happens, GCC doesn't manage to spot this through the call to fls(),
but the issue remains if architectures choose to implement their shift
instructions differently.

An example would be arch/arm/ (AArch32), where LSL Rd, Rn, #32 results
in Rd == 0x0, whilst on arch/arm64 (AArch64) LSL Xd, Xn, #64 results in
Xd == Xn.

Rather than check explicitly for the problematic shift, this patch adds
an extra shift by 1, replacing fls with __fls. Since zero_bytemask is
never called with a zero argument (has_zero() is used to check the data
first), we don't need to worry about calling __fls(0), which is
undefined.

Cc: Victor Kamensky <victor.kamensky at linaro.org>
Signed-off-by: Will Deacon <will.deacon at arm.com>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 include/asm-generic/word-at-a-time.h | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/include/asm-generic/word-at-a-time.h b/include/asm-generic/word-at-a-time.h
index d3909ef..d96deb4 100644
--- a/include/asm-generic/word-at-a-time.h
+++ b/include/asm-generic/word-at-a-time.h
@@ -50,11 +50,7 @@ static inline bool has_zero(unsigned long val, unsigned long *data, const struct
 }

 #ifndef zero_bytemask
-#ifdef CONFIG_64BIT
-#define zero_bytemask(mask)	(~0ul << fls64(mask))
-#else
-#define zero_bytemask(mask)	(~0ul << fls(mask))
-#endif /* CONFIG_64BIT */
-#endif /* zero_bytemask */
+#define zero_bytemask(mask) (~0ul << __fls(mask) << 1)
+#endif

 #endif /* _ASM_WORD_AT_A_TIME_H */
--
1.9.1